The Pentagon says Defense Secretary Robert Gates has ordered top officials to review security measures following the shootings earlier this month at Ft. Hood in Texas, in which a U.S. Army officer killed 12 fellow-soldiers and one civilian.

Pentagon Spokesman Bryan Whitman says Secretary Gates has met with top officials and told them to ensure the investigation into the Ft. Hood massacre is comprehensive, and also to look at whether there was anything the department could have done to prevent it.

"An incident like this obviously gives you the opportunity to pause and ask yourself, 'Are we doing everything we can and should be doing to address the range of issues that come out of an event like this," he said.

The Defense Department effort is part of a broad review President Obama has ordered. In his weekly radio and Internet address Saturday, he called the Ft. Hood incident "unthinkable."

"Given the potential warning signs that may have been known prior these shootings, we must uncover what steps - if any - could have been taken to avert this tragedy," said Mr. Obama.

The president said he met twice with senior officials in the hours after the shootings, and ordered them to conduct "a full review of the sequence of events that led up to the shootings."

"The purpose of this review is clear: We must compile every piece of information that was known about the gunman, and we must learn what was done with that information. Once we have those facts, we must act upon them," he said. "If there was a failure to take appropriate action before the shootings, there must be accountability. Beyond that - and most importantly - we must quickly and thoroughly evaluate and address any flaws in the system, so that we can prevent a similar breach from happening again," he added.

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Reports about the investigation say the alleged gunman, Major Nidal Malik Hasan, had contacts with a militant imam in Yemen and may have transferred money to some individuals or groups in Pakistan, where many militant groups operate. He also made a presentation at a medical conference that some now say hinted that he had militant views.

There are questions about how much Army officials knew about Major Hasan's alleged foreign contacts and political leanings, and whether they could have taken some action that might have prevented the massacre. President Obama said the U.S. government must be able to act decisively when information surfaces about a potential threat.

At the Pentagon, Bryan Whitman would not provide any details about what types of changes are being considered.